In recent years, application of infinite solar light producing no harmful substances has been actively studied. Inorganic type solar cells such as single crystalline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride and indium copper selenide for domestic use are provided as those presently available in practical use as a clean energy source in application of solar light.
However, as drawbacks of these inorganic type solar cells, in the case of the silicon type, no only extremely high purity is required, but also the complicated purification process includes many steps at high production coat.
On the other hand, many solar cells employing an organic material have also been proposed. Examples of the organic solar cell include a Schottky type photoelectric conversion element in which a p-type organic semiconductor and metal having a small work function are joined, and a heterojunction type photoelectric conversion element in which a p-type organic semiconductor and an n-type inorganic semiconductor or a p-type organic semiconductor and an electron acceptable organic compound are joined. The utilized organic semiconductors are synthesized dyes or pigments such as chlorophyll, perylene and so forth, conductive polymers such as polyacetylene and so forth, and the composite material thereof. Such the material to be used as the cell material is thin-layered by a vacuum evaporation method, a casting method, a dipping method or the like. The organic materials have advantages of low cost is low and easy production of large area, but there is a problem such as a low conversion efficiency of 1% or less together with insufficient durability.
In such the situation, a solar cell exhibiting favorable properties has been reported by Dr. Gratzel et al. in Switzerland, cf. Non-patent document 1 for example. The proposed cell is a dye sensitizing type solar cell, and a wet type solar cell in which a porous titanium oxide thin film spectrally sensitized by a ruthenium complex is provided as a functional electrode. Advantages of this technique are that purification of a cheep oxide semiconductor such as titanium oxide up to high purity is not necessary, and solar light having a large visible light component can be effectively converted into electricity in accordance with usable light covering a wide wavelength of visible light at low cost.
In contrast, ruthenium complex is under threat of its supply in cases where this solar cell is put into practical use, since the ruthenium complex as limited resource is utilized. Further, since ruthenium complex is expensive, and produces a problem in aging stability, this problem can be solved if the ruthenium complex can be replaced by an inexpensive and stable organic dye.
It is known that a dye molecule having both of a π-electron conjugated system exhibiting electron donating ability and an acidic adsorption group exhibiting an electron withdrawing property provides an element exhibiting high photoelectric conversion efficiency. Triarylamine derivatives have been widely used as the electron donating π-electron conjugated system (refer to Patent Documents 1-4, for example). However, there remains a problem such that absorption on the long wavelength side of visible light is weak, since a solution of this dye has an absorption peak of 500 nm or less. A photoelectric conversion element capable of more effectively utilizing solar light energy can be provided when expanding the light absorption range of the dye further to the longer wavelength side. As a means to expand the light absorption range to the longer wavelength side, longer wavelength via absorption through J-aggregate can be provided (refer to Patent Document 5, for example). However, since the durability level is not satisfactory, it is found out that the level is demanded to be further improved.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2005-123033
(Patent Document 2) Japanese Patent O.P.I. Publication
(Patent Document 3) Japanese Patent O.P.I. Publication
(Patent Document 4) Japanese Patent O.P.I. Publication
(Patent Document 5) Japanese Patent O.P.I. Publication
(Non-patent Document 1) Nature, 353, 737 (1991), B. O. Regan, M. Gratzel